The present invention relates to a non thermoprocessable fine powder of homopolymer or modified PTFE as defined below, and to the process thereof, via powder to be used in the lubricated extrusion wherein also low extrusion pressures can be used. The manufactured articles obtainable from said fine powders have a high purity degree and are surface defect free, such for example cracks, roughness.
It is known that the polytetrafluoroethylene manufactured articles are obtained in various forms, such for example, pipes, cables, wires or tapes, subjecting the PTFE fine powders to a lubricated extrusion process. Generally, a higher extrusion pressure leads to a defect increase of the extruded manufactured article, whereby it is always preferable to work I at a low extrusion pressure. Before extrusion it is necessary to subject the fine powders to drying. The extrusion pressure results depend on the temperature at which drying is carried out. The higher the drying temperature, the higher the polymer particle coalescence degree whereby it will be necessary a higher extrusion pressure. Therefore it is extremely advantageous to be able to dry the fine powder at a lower temperature, to work at low extrusion pressures. The PTFE fine powder obtained at lower drying temperatures results suitable to be extruded with a sufficiently low pressure, obtaining extruded manufactured articles surface defect free, such for example cracks, roughness.
It is known that the PTFE fine powders are obtained by the polymerization in dispersion (emulsion). In said process a sufficiently high amount of surfactant is used so to be able to stabilize the PTFE colloidal particles and a mild stirring is applied so to avoid the polymer coagulation. In the dispersion polymerization process stabilizers, initiators and other additives (for example nucleants such as ZnCl2) are added to control the polymer particle diameter. Then the latex obtained from said process is coagulated, and the obtained powder is called xe2x80x9cfine powderxe2x80x9d. The known conventional coagulation process comprises the following steps:
latex dilution with water and optional addition of a destabilizing electrolyte to coagulate the latex;
latex mechanical stirring which causes the aggregation of the colloidal particles and leads first to gelification then to granulation and lastly to flotation;
separation of the wet fine powder from the coagulum water;
optional polymer washing, under powder form, for several times with water to reduce the electrolyte concentration in the powder;
optionally drying of the fine powder.
In the dispersion polymerization process of PTFE, surfactants and polymerization initiators are commonly used, for example persulphates, which after the polymerization process remain in the latex. The polymer obtained from this conventional process is therefore contaminated by the initiator and surfactant. For said reasons the PTFE fine powders cannot be used for the porous manufactured article production, for example defect free thin membranes. Furthermore said fine powders cannot be used in the production of manufactured articles for the semicon industry. In fact, as well known, in the applications concerning the semicon industry, a high purity degree of the polymer is required, in particular having a content of residual cations  less than 1 ppm, and of anions (in particular sulphates  less than 1 ppm).
In U.S. Pat. No. 3,046,263 a continuous coagulation process of the PTFE latexes is described, comprising
a phase of strong mechanical stirring with a specific power of 1-100 (CVxc3x97sec)/gallon (196 kJ/m3-19,600 kJ/m3), preferably using a centrifugal pump with average residence time of the latex in the pump of 2 seconds;
passing through a capillary tube with hydraulic resistance of 0.5-20 p.s.i. (3.4-136 kPa);
granulation in the presence of air by mechanical stirring with specific power of 0.25-50 (CVxc3x97sec)/gallon (49 kJ/m3-9,800 kJ/m3) with subsequent separation of the fine powder from water.
The use of the mechanical stirring with so high specific powers (xcx9c98 kW/m3-9,800 kW/m3) by a centrifugal pump, which has a wide distribution of the residence times, causes a too compact structure of the fine powder, which does not mixes well with the lubricant and does not allow to work at low pressure in the lubricated extrusion process as described hereafter.
In U.S. Pat. No. 5,391,709 a purification process of the PTFE fine powders from contaminants is described, wherein a thin layer of fine coagulated powder is put on the surface of a polymer tissue and subsequently exposed to a hot air flow, which passes through the fine powder layer flowing from the upper part to the lower part. In said process the drying temperature is in the range 110xc2x0-200xc2x0 C., preferably 160xc2x0-200xc2x0 C. Tests carried out by the Applicant (see comparative Examples) show that by operating under the conditions described in this patent, a product free from initiator salts is not obtained; in fact the residual cation content is higher than 1 ppm, the residual anion content (sulphate) is higher than 1 ppm. Therefore the purification process described in said patent is not suitable to obtain manufactured articles usable in the semicon industry, which requires a residual cation content  less than 1 ppm and a residual anion content (sulphate)  less than 1 ppm. Besides, said patent refers to PTFE fine powders coagulated with the conventional coagulation process, as above described. Tests carried out by the Applicant (see comparative Examples) show that by subjecting said PTFE fine powder to the purification process indicated in said patent wherein the drying temperature is lower than 160xc2x0 C., a product still containing some surfactant is obtained. Therefore the manufactured articles obtainable under said conditions have not the purity requirements to be used in the semicon industry. In fact it is necessary to work at a drying temperature of 190xc2x0 C. to obtain a product purified from the surfactant. However in the extrusion phase the purified product according to said patent requires high extrusion pressures since the drying temperature is high. Therefore the purification process described in said patent does not allow to obtain a fine powder which can be subjected to low extrusion pressures.
The need was felt to have available non thermoprocessable fine powders of homopolymer or modified PTFE as defined below, which can be subjected to subsequent extrusion using low extrusion pressures. The obtainable manufactured articles have a high purity degree, are surface defect free, such for example cracks and roughness, and are used for the application in the semicon industry.
The Applicant has surprisingly found a purification process of homopolymer or modified PTFE which allows to obtain PTFE fine powders with the above advantages.
It is therefore an object of the present invention a process to obtain non thremoprocessable fine powders of homopolymer or modified PTFE, comprising the following steps:
A) obtaining of the polymer latex under the gel form;
B) washing of the polymer gel with acid aqueous solutions or neutral aqueous solutions, having a pH from 0.5 to 7, preferably from 1 to 4;
C) granulation of the washed gel by mechanical stirring, at a specific power in the range 1.5-10 kW/m3, until flotation of the PTFE fine powder, and separation of the floated fine powder;
D) drying in an aerated oven of the floated fine powder at a drying temperature in the range 90xc2x0 C.-160xc2x0 C., preferably 105xc2x0 C.-150xc2x0 C.
The process of the present invention allows to obtain PTFE fine PTFE powders suitable for low pressure extrusion. The powders ar substantially free from inorganic cations and from surfactants.
The fine powders of PTFE or PTFE modified after step D), as said, are substantially free from inorganic cations (residual amount  less than 1 ppm), substantially free from inorganic sulphates (residual amount  less than 1 ppm), and contain polymerization surfactants in an amount lower than the analytical detectable limits ( less than 10 ppm, determined as indicated in the characterization methods).
Unexpectedly it has been found that by the process of the invention the polymerization surfactant is easily eliminated during the drying step D) at low temperature.
The process of the present invention can be carried out in batch or in a continuous way on PTFE or PTFE modified latexes as defined below.
When the process of the present invention is carried out in batch, step A) for obtaining the polymer latex under gel form comprises the following steps:
dilution of the latex obtained from the polymerization in dispersion a concentration from 2 to 25% by weight of PTFE, preferably from 8 to 20% by weight of PTFE; the dilution being carried out by addition of water at a temperature such that the temperature of the diluted latex is from 5xc2x0 C. to 35xc2x0 C., preferably from 15xc2x0 C. to 30xc2x0 C.;
optionally, the filtration of the diluted latex is carried out to remove the particle aggregates in case formed;
subsequent latex mechanical stirring using a specific power from 1.5 to 10 kW/m3;
addition of an acid electrolyte, preferably nitric acid, until obtaining a dispersion having pH from 0 to 3.5, preferably from 1 to 3;
keeping of the mechanical stirring at the specific power from 1.5 to 10 kW/m3, until gel formation.
By gels it is meant that the polymer particles are dipped in the liquid phase and linked by crossed bonds so to form a thick network. The gel properties significantly depend on the interactions of said two components (polymer and liquid). In fact the retained liquid prevents the polymer network from being transformed into a compact mass and the network prevents the liquid from coming out from the gels. Depending on the chemical composition and on other process parameters, such for example the solid and electrolyte concentration, the gel consistence can vary from a viscous fluid to a rather stiff solid.
In the batch process, when the polymer has been obtained under the gel form, the subsequent washing step B) comprises the following steps:
1) mechanical stirring stop and addition of an aqueous solution as indicated in B). The added aqueous solution amount is generally from 100 to 200 parts by volume of solution for 100 parts by volume of polymer gel;
2) subsequent gel mechanical stirring using a specific power generally in the range 0.2-2 kW/m3, for a time generally from 1 to 10 minutes; under said conditions the gel is shattered into small masses, but at the same time the contact between gel and air is minimized, obtaining flocks which maintain the hydraulic contact with water without floating;
3) stirring stop, gel flock decantation and removal of the supernatant water.
Steps 1)-3) are repeated until complete removal of the inorganic cations from the gel phase, i.e. until a residual amount of cations lower than 1 ppm. In practice after step 3) the residual cations are substantially absent. It has been found that also the residual sulphates are substantially absent ( less than 1 ppm after step 3). The washing step B) is repeated from 1 to 10 times, preferably from 3 to 8 times.
When the gel washing is carried out until substantial removal of the residual cations, one proceeds with the subsequent granulation step C) of the washed gel, comprising the following steps:
mechanical stirring of the washed gel by using a specific power from 1.5 to 10 kW/m3, optionally adding other acid electrolyte; the gel temperature is from 5xc2x0 C. to 35xc2x0 C., preferably from 15xc2x0 C. to 30xc2x0 C.; the mechanical stirring is continued until the complete granulation and flotation of the fine powder;
stirring stop and separation of the aqueous phase underlying the floated fine powder.
The obtained fine powder is subjected to drying step D) in an aerated oven. The oven must be resistant to acid vapours.
As said, the process of the present invention can be carried out also in a continuous way. In this case the polymer latex under gel form (step A) is preferably obtainable through the following steps:
a1) dilution in a lift of a PTFE latex obtained from the polymerization in dispersion (emulsion) to a concentra-from 5 to 25% w/w of PTFE, optional filtration of the diluted latex,
b1) latex pressurization in the lift by an inert gas, until a pressure, referred to the atmospheric pressure, in the range 3-40 kg/cm2 (0.3-4 MPa),
c1) addition of a solution of an acid electrolyte, preferably nitric acid, to the PTFE latex, in a in-line mixer, so that the pH is from 1 to 4,
d1) latex flowing from the mixer through a capillary tube for the gel obtaining.
When the polymer under the gel form has been obtained, one proceeds to the subsequent washing step B) as above described in the case of the batch process.
To carry out the process in a continuous way, at the end of step b1), while the previously pressurized latex is discharged in the mixer of step c1), a second lift is used which is fed with latex to be diluted according to step a1). When the first lift has been emptied, the latex fed in the second lift is at the end of step b1), and therefore the latex is fed again in the first lift.
As said, fine powders of homopolymer and modified PTFE as defined below, obtained by the process of the present invention, by lubricated extrusion using low extrusion pressures, allow to obtain manufactured articles free from inorganic contaminants and from surface defects. Therefore, the obtained manufactured articles can be used in applications wherein a high purity of polytetrafluoroethylene or modified PTFE is required, such for example in the semicon industry.
In the case of non thermoprocessable fine powders of modified PTFE, the polymer contains small amounts of comonomers having at least one unsaturation of ethylene type in an amount from 0 to 3% by moles, preferably from 0.01 to 1% by moles.
The comonomers which can be used are of both hydrogenated and fluorinated type. Among hydrogenated comonomers, ethylene, propylene, acrylic monomers, for example methyl methacrylate, (meth)acrylic acid, butylacrylate, hydroxyethyl hexylacrylate, styrene monomers, such for example styrene, can be mentioned. Among fluorinated comonomers it can be mentioned:
C3-C8 perfluoroolefins, such hexafluoropropene (HFP);
C2-C8 hydrogenated fluoroolefins, such vinyl fluoride (VF), vinylidene fluoride (VDF), trifluoroethylene, hexafluoroisobutene, perfluoroalkylethylene CH2=CH-Rf, wherein Rf is a C1-C6 perfluoroalkyl;
C2-C8 chloro- and/or bromo- and/or iodo-fluoroolefins, such as chlorotrifluoroethylene (CTFE);
(per)fluoroalkylvinylethers (PAVE) CF2=CFORf, wherein Rf is a C1-C6 (per)fluoroalkyl, for example CF3, C2F5, C3F7;
(per)fluoro-oxyalkylvinylethers CF2=CFOX, wherein X is: a C1-C12 alkyl, or a C1-C12 oxyalkyl, or a C1-C12 (per)fluorooxyalkyl having one or more ether groups, for example, perfluoro-2-propoxy-propyl; fluorodioxoles, preferably perfluorodioxoles;
fluorovinylethers (MOVE) of general formula: CFXAI=CXAIOCF2ORAI (A-I) wherein RAI is a C2-C6 linear, branched or C2-C6 cyclic (per)fluoroalkyl group, or a C2-C6 linear, branched (per)fluorooxyalkyl group, containing from one to three oxygen atoms; when RAI is a fluoroalkyl or a fluorooxyalkyl group as above it can contain from 1 to 2 atoms, equal or different, selected from the following: H, Cl, Br, I; XAI=F, H; the compounds of general formula CFXAI=CXAIOCF2OCF2CF2YAI (A-II), wherein YAI=F, OCF3; XAI as above are preferred; in particular (MOVE I) CF2=CFOCF2OCF2CF3 (A-III) and (MOVE II) CF2=CFOCF2OCF2CF2OCF3 (A-IV) are preferred.
The process of the invention is highly effective, since the polymer losses in the purification process carried out according to the present invention are negligible, of the order of 0.1% by weight of PTFE.
Among the preferred acid electrolytes, nitric, hydrochloric acids can be mentioned, nitric acid is preferred.
As said, the drying oven must be resistant to acids. The drying temperature is preferably from 105xc2x0 to 150xc2x0 C. The productivity in the process of the present invention can be increased by increasing the thickness of the powder put on the support for drying. The Applicant has surprisingly found that said thickness increase does not cause coloration problems or anyway a worsening of the purification process. Used thicknesses can also be of 5-6 cm.
In drying step D), among the preferred used substances which are put at direct contact with the PTFE fine powder, substances resistant to nitric acid in the temperature range from 105xc2x0 C. to 150xc2x0 C. can be mentioned. Fluoropolymers such PTFE, PVDF, FEP (optionally modified with vinylethers), PFA, MFA, or PEEK, can for example be mentioned. Other hydrogenated polymers such as for example PET and PPS have not been found usable.
The polymer latex from which gels are obtained as above described, is obtained from polymerization in dispersion (emulsion) of TFE, optionally in the presence of comonomers as said above. The primary particles of the latex polymer have sizes from 0.1 to 0.4 micron. The process for the latex obtaining can also be carried out in microemulsion. See for example U.S. Pat. No. 4,864,006, U.S. Pat. No. 4,990,283 and EP 969,027. In this case the diameter of the primary particles of the latex ranges from 0.01 to 0.1 micron.
The present invention will now be better illustrated by the following embodiment Examples, which have a merely indicative but not limitative purpose for the scope of the invention itself.